In the state of the art electrospray ionisation ESI is known. Electrospray ionisation ESI and MALDI (matrix-assisted laser desorption/ionisation) with ultraviolet (UV) and infrared (IR) lasers can be used in combination with any mass spectrometer means, for example on an ion trap mass spectrometer. Recent developments in other laboratories include DESI (desorption ESI), DART (direct analysis in real time) and EESI (extractive ESI). In the first two methods either an electrospray or a stream of gas containing excited gas molecules (of e.g. He) and ionised water clusters, are used to desorb and ionise material from a sample at atmospheric pressure. The third method employs post-ionisation of desorbed molecules in a secondary ESI process.
Throughout the description molecules will be understood as neutral, i.e. uncharged species while ions are molecules carrying at least one charge. Ions can be desorbed from the sample when they already exist as ions in the sample or can be desorbed/ionised (i.e. desorbed and ionised) from the sample. In the latter case of the direct generation of ions from uncharged molecules the processes of desorption and ionisation are intertwined and shall be summarized as desorption/ionisation throughout the description. Alternatively, a post-ionisation means can however be used to ionise non-charged molecules that are desorbed simultaneously or exclusively.
In another related technique termed PESI (probe ESI) a solid needle is covered with a drop of sample solution which is then electrosprayed. In other related techniques gas phase molecules are first generated by desorption by any suitable technique, for example by electrospray or by laser desorption, and subsequently post-ionised. State of the art post-ionisation means are, for example, ionisation through interaction with ionising chemical agents, CI (chemical ionisation) and APCI (atmospheric pressure chemical ionisation), PI (photon ionisation; by interaction with a beam of photons) and APPI (atmospheric pressure photoionisation), and EI (electron ionisation) by interaction with a beam of electrons or EESI.
The comparatively old techniques of field desorption FD/field ionisation FI are also related to the invention, although such ion sources are operated in a high vacuum rather than at atmospheric pressure as in the invention.